This invention relates generally to operational amplifiers, and more particularly, to an all NPN monolithically integrable output stage for an operational amplifier which includes a thermal current source.
To achieve a higher frequency response, greater output swing, reduced output stage emitter-follower peaking and to simplify integrated circuit construction, operational amplifiers have been developed which include output stages incorporating only NPN transistors. Such an output circuit is shown and described in U.S. Pat. No. 4,403,200 entitled "Output Stage for Operational Amplifier". Unfortunately, as was the case with prior art operational amplifiers, the circuit can become unstable at low temperatures. This is true since the gain of the output Miller loop is directly proportional to the ratio of the load resistance to the dynamic emitter resistance. As the temperature drops, the dynamic emitter resistance similarly drops causing a significant increase in the gain thus rendering the amplifier unstable.